Signal translating apparatus



Patented Sept. 14, '.1937

UNITED STATES PATENT rol-FICE y 2,092,884 SIGNAL TRANSLATING APPARATUSlJames M. Kendall, Woodbury Heights, N. J., as-

slgnor to Radio Corporation of America, New York, N. Y., a corporationof Delaware Application November 1, 19.32, Serial No. 640,591 l Thepresent invention relates to signal translating apparatus of the typeemployed in electrical phonograph sound recording and reproducingsystems, wherein the sound record is provided in a mechanical groove orother sound.

character above referred to, a vibratory system isY provided,comprising, in one Well known type, an amature adapted to carry a stylusfor engagement with a record groove and pivotally mounted to vibrate atone end between magnetic pole pieces in association with an electricalwinding or pickup coil.

The vibratory mechanism or system, of which g5 the armature `is a part,may have one 'or more natural periods of vibration within the audiblerange at which the amplitude of vibration may be over-'emphasized orunder-emphasized, resulting in distortion of signals being translated bythe device. Obviously this is undesirable. As indicated, 'for example,by the construction shown in the patents above referred to, variousmeans have been provided in the past for correcting for such conditionsof operation such as damping the armature action to effectively smooththe operating characteristic as represented by the usualfrequency-voltage response curve to remove the undesired peaks orresulting dips therein caused by resonance. In general, the prior artdevices employ damping and centering means comprising blocks ofresilient material engaging the amature on opposite sides and arecharacterized by the fact that the damping means (1) Is interposedbetween and connects the armature and a fixed supporting means,

' (2) Is both the-damping and the centering or restoring means,

- (3) Has a limited damping'action, because of rigidtyand (4) Undulyloads the armature,l`

whereby the latter is less flexibly mounted and is less responsive inthe translating action because of the connection with the nxedsupporting means.

05 The damping action is limited because of the material employed,usually soft rubber, and because the damping means is so mounted that itattenuates desired vibrations because of its rigidity. Furthermore, vtherubber ages and must be replaced occasionally for best results in opera-5. tion,'and while providing damping to a certain extent, does notprovide enough entirely to eliminate certain peaks in the audiofrequency response characteristic of electric pickup devices for highfrequency electric signal translation. 10 .It is, therefore, a primaryobject of the present invention to provide an improved signaltranslating device for electric phonograph sound recordingandreproducing systems, that shall have an improved frequency responsecharacter- 15 -istic and that shall obviate the above mentioned andvcertain other disadvantages of known devices.

It is a further object of the present invention to provide an improvedmounting, centering and damping system for the vibratory element of adevice of the above character that shall eectively attenuate undesiredresonant peaks inthe operating characteristic thereof, without loadingthe armature orvibratory system appreciably outside the frequency rangeof such peaks, whereby' the frequency response characteristic may beimproved. l

In accordance with the invention, in a preferred embodiment thereof, avibratory member 30 or armature is separately mounted in fixed pivotbearings with a torsional restoring or centering spring extendingtherefrom along the pivot axis, separately connected to a xed, spacedsupporting frame. The armature member is provided 35 `with a body ofdampingmaterial directly se-,

cured or connected thereto. The damping' body is free to vibrate and tomove with the armature since it has no other support, and at least oneof'` its largest dimensions is parallel to the plane of 40 vibration ofthe armature member, the plane of vibration being at a right angle tothe pivot axis of the armature..A l l The-shaft bearings have theseparate function 1 of providing, with the pivot shaft, a fixed, inde-45 pendent, pivot axis for the armature-member. l' The fixed torsionalspring, which may have a low l hysteresis effect as hereinafterdiscussed, provides a restoring and centering force without appreciablyaffecting the moment of inertia of the 50. vibratory system, since itlies in the axis of vibration. The damping means has the separatefunctionof damping the armature, and, being free to move with theamature, .may be shaped to selectively remove undesired higher frequency,55

peaks in the operating characteristic of the device without having Vanyappreciable damping effect at other and lower frequency ranges. By thisconstruction, furthermore, the use of resilient 5 material of anon-permanent nature, such as rubber, is substantially eliminated and isemployed only as a friction reducing means as a part of the shaftbearing for the armature.

It is a still further object of the present invention to provide animproved electromagnetic pickup device ofthe type adapted to be providedwith a stylus, wherein the vibratory element or armature is moreflexibly mounted, thereby to reduce record wear and to improve thefrequency response characteristic.

Contrastedwith the selective frequency responsive damping means abovementioned, the torsional centering spring arrangement provides forrestoring the vibratory element to a centralncrmal position with astiffness that is constant within the audio frequency band. No resonanceand a very low moment of inertia are the characteristic of a restoringmeans of this character, and by properly choosing the material, very lowmechanical hysteresis is obtained.

Hysteresis in vibratory systems is the equivalent of friction, so thatwhen the vibratory element or armature is deflected from its normalposition, it does not return exactly thereto, but only to apoint wherethe centering force is balanced to a force that is necessary to overcomethe friction. The greater the hysteresis the more oif center will thearmature be after it is deflected. In electric signal translatingdevices of the .present electric pickup type,v the amature, ,in itsunexcited condition, stands normally in a central position with one endmidway between the two pole pieces. this character, in operation, thearmature is deflected from the central position either by mechanicalforces or by forces derived from electric currents owing in the coil.

- If rubber damping blocks are employed' in connection with the armatureelement for restoring the armature to and for normally maintaining it ina centered position, it has been found that the hysteresis effect abovementioned is relatively high. Accordingly, to overcome the effects ofhysteresis in such a case, it has heretofore been necessary to provide arestoring force in the form of a spring in certain cases, or to rely onthe rubber only, the spring being thereby stiffened to overcome thehysteresis effects.

In a translating device embodying the present invention, the dampingmeans is carried entirely by the moving or vibratory armature whereby itimparts no fixed rigidity thereto and, accordingly, lthe restoring forceor spring may be reduced to a point which makes -it possible to providea 00 flexible armature element or stylus carrying member and therebyappreciably to reduce record wear. In this position, the responsecharacteristic of the device is improved because of the flexibility ofthemoving parts. l 6 The invention will, however, be better understoodfrom the following description when taken in connection with theaccompanying drawing, and itsscope will be pointed out in the appended75 Fig-2 is a cross sectional view taken on line As is well known withdevices of is a front view. on a' y is rigid against 2-2 of the deviceshown in Figafl, on Ithe sameV .scale and with a cover plate in place;

of the devices of Figs. 1, 2, and 3, showing modi` fications; and

Fig. 6 is acurve diagram illustrating an operating characteristic of thedevice shown in Figs. l, 2, and 3.

Referring to Figs. 6 is the molded insulation base of an electromagneticpickup-device on which is mounted a metallic frame 'l providing magneticpole pieces 8 to which is connected a permanent magnet 9 f or supplyingflux thereto. An armature or vibratory member i0 provided with anintegral pivot shaft ii is journalled in the frame i in suitable spacedbearings provided by rubber sleeves i'Z-IZ mounted on the shaft andclamped with it between two frame members i3.

In the present example, the armature is hollow at one end and may beprovided with a suitable stylus i4, which is clamped within the hollowend of the armature by a clamping screw or' thumb screw I5, threadedaxially into the shaft I i. The opposite end I6 of t'ner armatureis'ada'pted to lie between the pole pieces 8 and is surrounded by asuitable electric pickup winding or coil I'I provided with terminalleads I8 which, in turn, are connected to output terminal members i9embedded in the body 6 of the pickup device for external connection withan electrical circuit (not shown).

The permanent magnet 9 is held in place on the frame 1 by a. springretaining member 20 connected with the body 6 in a position to engagethe top of the magnet. -The pickup device is `also provided 4with anexternal main cover 2i, all as shown more clearly in Fig. 2.

It will be noted that the vibratory member or armature is pivotallymounted in the .bearing sleeves |2-I2 which, as hereinbefore stated, arepreferably of rubber, whereby friction is avoided or reduced to a Thebearings are y spaced, and it will be noted that the armature member isthereby prevented from `movement in the radial directions.v Thisarrangement prevents the weight of the device on the stylus fromdisplacing the armature member. In other words, vthe armature member isIjournalled in means which function exclusively as pivot bearings, toconstrain the armature to rotary motion without radial movement. y

The armature member is retained in a central or normal position withrespect Vto the pole pieces 8, by an elongated spring' member ortorsional restoring and centering spring 22 which lies in axialextension of the pivot shaft Il at one end thereof and which isconnected with said end and with a spaced bracketI member 2E formingpart of the frame 1. The spring may be either round or rectangular incross section. In the present example, the spring issubstantiallyrectangular in cross section and' is dat at its endswhereby it may be secured in holes or slots i'n the shaft end'and in theframe extension' or bracket'member 23.

The bracket member 23 and the spring are preferably united permanentlyby suitable means auch as by soldering (not shown), whereby the springis rigidly connected with the frame and movement in{any direction ex-.v

'cept torsionally, that is, about its own 'sais 1, 2, and 3 of thedrawing,`

and that of the shaft Il. The spring is secured to the armature by anysuitable means, such as by slot 24, in one end of the shaft I I, over.which is forced a sleeve 25.

By this arrangement, it will be seen that the vibratory member orarmature is not only rigid-v ly constrained to rotary motion by spacedbearing means provided in a rigid frame, but it is also connected withsaid frame by further means comprising a torsional spring which servesto prevent longitudinal movement of the shaft and armature within thebearings should there be any tendency for such movement, and which alsoholds the armature resiliently centered between the pole pieces; Thetorsional spring connection between the armature and the frame is ofsuch cross section, although relatively small since it is preferably ofsteel, that it may impart sufiicient centering action to properly returnthe armature to and normally hold it in the centered position. Y

It will also be noted that the frame and the adjacent shaft end arerelatively widely spaced, whereby the torsional spring may be relativelylong, thereby reducing the stresses in the spring material. Thestiffness of the spring is substantially constant and it is non-aging ascom` 'pared with rubber as a restoring means for the armature. Being ofsteel and of xed dimensions it may be made very uniform in quantityproduction. Furthermore, it is rigid in all directions, excepttorsionally, and has no resonance in itself. Because it liesin the axisofthe armature pivot shaft, it has substantially no appreciable inertiaor -resonance in itself and has no appreciablemechanical hysteresis.

The sleeve 25 is arranged to exten'd'in the direction of the spring 22into the space Abetween the frame member 23 and the main portion of theframe, and is there enlarged to form the cylinder 26 on which is mounteda block -of damping material 21. `In the present example,

the block of damping material is rectangular in shape and is relativelythin, having at least one of its largest dimensions in a plane parallelto that of vibrational movement of the armature, that is, at a rightangle to the shaft axis. The exact dimensions of the block 21 are notcritical. It is desirable, however, that the moment of inertia thereof,about the axis of vibration of the armature member, shall be greaterthan the moment of inertia of the armature member. about the same axis.l The function of the sleeve or cylinder jis to providea rigidconnection between theVl` block of damping material 21 and the armature,thereby to impart to the mass of damping material,

through the cylinder, and in the plane of one of its larger dimensions,a predetermined vibratory force corresponding to that to which theamature member is subjected at its resonance peaks. The damping block issubjected to rotational and torsional movement in the example shown, butit may be attached to the armature by other means, as indicated in Fig.4 for example, to which attention is now directed.

In Fig. 4, the armature member I0 is provided e with a cylindricalextension piece Illaon which is mounted a relatively large block ofdamping material 21a. The damping action is similar to that provided bythe block 21 in Fig. v3 and the material and the damping action per sewill be further considered hereinafter, Fig. 4 being illustrative, alongwith the precedingfivgures, of an arrangement of the damping means inconnecor the cylinder 26 of Figs. 2 and 3. In general,

friction may be depended upon .for this connection, `or in the caseof afusible material, friction, supplemented by fusion', yproduced bysuitably heating the cylinder 26, for example by a soldering iron or thelike, until the material of the block fuses and thereby more rmlyadheres to the cylinder 26. In the case of non-fusing materials, theseparts may be cemented together to supplement the frictional connection.

In certain cases, however, it will be desirable to provide additionalmeans, as shown in Fig. 5, for example, for insuring that the block doesnot slip or twist `on its supporting mediumand to this end a cylinder29, corresponding to cylinder 26 of Fig. 3, may be provided withradially extending fins 30, which will cut into and, therefore, grip thedamping material when the damping block is pressed onto the cylinder. l

'I'he damping material found most suitablefor the use shown, is a soft,pliable and solid substance, such as that known vcommercially asGlyptal, or another commercial product known as Du Pont Viscoloid,X'17'15-3.

'Ihe material should preferably have all the properties of a solid, butshould be inert and almost fluid, while being self supporting or adaptedto hold a permanent shape.

Specifically, the damping material preferably has the followingproperties and characteristics:

Masa-The important thing about mass is that it has inertia,'orresistanceto change of velocity. The materials at present foundmost'satisfactory j vhave a density of about 1, but this value does notappear to be critical.

Rz'gz'dity. -One of the differences between ya solid anda liquid. Thematerial should have sufficient rigidity and mechanical strength to makeit self supporting. Too much rigidity adversely affects the performancein a translating device.

Damping.-Ordinarily, most easily thought of.

as a property ofheavy oil. 'I'he resisting force is proportional to thevelocity. (In a mechanical system it is similar in its effect toresistance in an electric circuit). .In regard to solids, it is oftenconfused with hysteresis, but it relates to hysteresis as eddy currentsdo to magnetic hys- The operation of the vibratory system em? bodyingthe invention is as follows: y

'I'he damping mass orblock is free to rotate With the armature when thearmature is slowly deflected in a normal manner. If, however, thearmature `is suddenly deflected, or shock excited, as in normaloperation, the outer edge portions of the mass or block will tend to'I'he present preuse in a systemv stand still at first.

have rotated through an angle corresponding to the angle through whichthe armature was suddenly deflected. The inner portion of the mass orblock adjacent to the cylinder obviously must rotate with the armaturesince it is directly connected thereto, The motion of the outer edgelags behind the motion of the inner or center portion under theseconditions, and thus the material is worked or deformed, when thedeflection is l required to oscillate the damping block di' mass.

The damping and rigidity of the vmaterial are sumcient to transmit thisforce from the cylinder t the outsideedge of the block or mass withoutdeforming the material. Hence, the entire block or mass oscillates inphase with the armature in a low frequency range.

At a, higher frequency range, the damping and rigidity of the materialare not sufficient to transmit to the edge of the block the increasedforce necessary to overcome the increased mass reactance at the higherfrequency and when the material is deformed, the portion near thecylinder or sleeve vibrates therewith, while the edge portions tend tostand still, thereby causing the damping and rigidity of the material tobecome effective and to control resonance effects in the higherfrequency range.

reproduction, and should be eliminated if possible. By making thedamping and rigidity of the damping means effective at a somewhat lowerfrequency, this resonance may be largely suppressed, thus improving thequality of reproduction.

The advantages inherent in the free selective damping system thusprovided may further be seen after a brief consideration of a ,systemwherein the damping material is bound or rigidly held, as in certainprior art devices. If, in accordance with such systems, the dampingblock were held by its outer edges, while the cylinder oscillates, thevclamping material is worked at all frequencies.V 'I'his has theobjection of introducing the damping and rigidity at frequency rangeswherein it is not required, thereby putting an unnecessary load on theneedle point, with a consequent increase in record and needle wear. Thisis especially true since the major portions of recorded sounds appear ina lower rather than a higher frequency range. If the load on the Vneedleis light, or in other words, if the mechanical impedance is low, in thislower range, as is the case with a construction like that of the presentembodiment of the invention, a major portion of what is usuallyconsidered normal record wear is avoided. 'I'he preferred arrangementis, therefore; to have the outer peripheral portion or edges 2,092,584.and then begin to rotate rather slowly until they of the damping blockor mass free and the shape and mass so adjusted as to make the dampingeffective at a frequency somewhat below the usual resonance.

The preferred embodiment, therefore, includes a torsional type damperwhich replaces the customary damping blocks on the tall of the armature.l'

If loading of the armature is unobjectionable, in certain cases it maybe desirable to increase the moment of inertia of the damping block asby adding to the mass at the edges by the same or heavier material, orby other suitable meansV such as clamping the edge of the block by ascrew 35. for example.

The effect of the damping introduced is to control the armatureresonance. The effect of the rigidity inherent in the damping materialvon the operation is to extend the frequency range. The upper limit ofthis range depends primarily on the moment of inertia of the armatureand the stiffness of the stylus or needle. In general, the greater thestylus stiffness and the smaller the moment of inertia of the armature,the greater the range. To a lesser degree, it is also true that thegreater the centering stiffness, the greater the range. The rigidityinherent in the damping material at high frequencies is added to thecentering stiffness, thereby increasing the range. Furthermore, indesigning the pickup, the diameter of the cylinder and the thickness ofthedamper'block are.

adjusted to give the best frequency response. In I adding the least massto the armature, because the cylinder is located coaxially with centerof oscillation of the armature. This makes its moment of inertia minimumand makes possible a design with better performance.

It has been found that the construction in aocordance with the presentinvention, as shown and described, has an improved range of operationextending effectively to 4500 cycles as compared with previousperformance of known devices of 3500 cycles.

From the foregoing description, it will be seen that a signaltranslating device in accordance with the invention comprises avibratory element through the movement of which the translating actionis effected and on which is mounted, or with which is rigidly connected,a block of material especially chosenI for damping characteristics, theshape of which and the mounting means for which is determined by thefrequency range 4'within which damping is desired.

It will also be seen that, in response to low frequency vibrations, thedamping block vibrates cient inertia to hold them substantiallystationary, while the inner portion adjacent to the hub or cylindervibrates relatively thereto permitting the absorption of the excessenergy in the frequency range to which the-vibratory system isresponsive.

Thus, it will be seen that the damping means,

instead of acting throughout the full audio frequency range, isselectively responsive and may K properly be termed a 4selective dampingmeans.

Referring now to Fig. 6, the electrical voltageoutput frequencycharacteristic of the pickup device shown in Figs. 1, 2 and 3 is plottedin two curves 3| and 32. These curves are not plotted throughout thelower frequency range, but only for the higher frequency range in whichcorrection for distortion is desired in the particular type oftranslating-device referred to.

'I'he curve 3l indicates the operating characteristic without employingthe damping block 21, Aand it will be noted that this characteristicindicates distortion in the region of 3500 cycles, while curve 3 2,showing the operation when provided with the damping block 21, is freefrom any appreciable -peaks of that character. This in. dicates animproved or extended higher frequency range, or an improved performance,and shows that the resonance characteristic of the vibratory system iscompensated for. By varying the size and the shape of the block, thedamping characteristic may, of course, be varied and may be caused toprovide any degree of correction between the curves indicated fory theparticular example herein given. For example, if the armature is loaded,as by increasing the inertia of the edges ofthe damping block bymeanssuch as that hereinbefore mentioned, while the mechanical impedanceis increased, the frequency characteristic is not' appreciablyaii'ected, as indicated by the curve 33 taken with the armature soloaded.

It should be understood, however, that the invention is not limited toelectromagnetic pickup 30 devices, although the vibratory system of thepresent example represents the present pre-- connected with saidarmature shaft, and having 4;, its major portion to one side of the axisof vibration of said shaft to provide a center of mass eccentric withrespect to said axis and a higher moment of inertia than the armaturemember about the axis of vibration of said shaft.

2. In an electric signal translating device, the combination with avibratory armature member of a frequency responsive selective dampingmeans therefor including a free mass of damping material rigidlyconnected with said arma- 55 ture member, and having its maior portionto one side of the axis of vibration 4of said member to provide aIcenter of mass eccentric with respect to said axis and a higher momentof inertia than the armature member about the axis of vibration i 60 ofsaid member, a supporting frame, a torsionally iiexible spring barextending axially of and from the armature to the frame and providingtherewith a rigid connection for the armature.

3, In an electric translating device, the combi- 65 nation with avibratory armature member of a frequency responsive selective dampingmeans therefor including a normally free mass of damping materialrigidly connected with said armature member and having its major portion70 extending to one side of the axis of vibration there-of, and meansassociated with said major portion for variably applying a load to saidmass of damping material at will.

4. In an electric signal translating device, the

75 combination with a vibratory armature member,

of a block of damping material having a direct connection only with andsurrounding an extension of the armature member to move there- -with asa unit in response to vibrations within a predetermined frequency range,said damping.

material having the characteristic of offering to an' external forcetending to distort it a resisting force which is proportional to thevelocity of distortion whereby it moves only in part adjacent to .andVin a region surrounding said extension of the armature member freely inresponse to vibrations within a second frequencyrange. n

v5. In an electric pickup device, the combination of a supporting frame,an armature membe'r journalled in said frame to vibrate about a i-lxedpivot axis, an elongated torsional spring member having substantially nomoment of vinertia about said axisconneoted with the shaft and extendingaxially therefrom into rigid connection with a portion ofthe frame, anda block of vibration damping material rigidly connected with thearmature member to vibrate therewith about said pivot axis.

6. In an electric pickup device, 'the combina# tion of a supportingframe, an armature member journalled in `said frame to vibrate about afixed pivot axis, an elongated torsional spring member connected Withtheshaft and extending axially therefrom into rigid connection with aportion of the frame, a block of vibration damping' material rigidlyconnected with the armature member to vibrate therewith about said pivotaxis, and a cylinder surrounding said torsional spring member andproviding said connection between the armature and the block of dampingmaterial.

7. In an electric pickup device, the combination of a supporting frame,an armature member journalled in said frame to vibrate about a fixed Ipivot axis, an elongated torsional springA member having substantially.no moment of inertia about said axis connected with the shaft andextending axially therefrom into rigidv connection with a portion of theframe, and a block of vibration damping material rigidly connected withthe armature member to vibrate therewith about said pivot axis, theblock of damping material having at least one of its greater dimensionsin the plane of vibration in which lit moves, and having a mass suchthat it has a higher moment of inertia about said pivot axis than thearmature member.

8. In an electric pickup device, the combination of a supporting frame,an armature member journalled in said frame to vibrate about a fixedpivot axis, an elongated torsional spring member having substantially nomoment of inertia about said axis connected with theshaft and extendingaxially therefrom into rigid connection with a portion of the frame, anda block of vibration damping material rigidly connected with thearmature member to vibrate therewith about said pivot axis, the block ofdamping material having at least one of its greater dimensions in theplane of vibration in which it moves, and said block being freelymovable as a unit with the armature member within a predetermined lowfrequency range and having a mass such that it has a higher moment ofinertia about said pivot axis than said armature member.

9. In an electric signal translating device, the combination with avibratory armature member, of means providing a fixed pivot axis for thearmature member, a torsional spring member connected at one end withsaid armature member and extending therefrom along said pivot axis, a

xed supporting means for the armature member rigidly connected with theopposite end of said spring member, said spring member having a lowermoment of inertia than the armature niember about said pivot axis, and ablock of damping material connected with the armature to move axis, anddamping means for said amature member having a common axis of rotationthere- With, said damping means including a body of damping materialconnected with said. amnature and having its major portion extending toone side of said aids whereby said damping means has a moment -ofinertia about said axis greater than that of the armature.

11. An electric pickup device including in 'combination, a pivotallymounted armature member, torsional damping means for the armature andcentering means for the armature member, said vcentering means extendingaxially along the pivot axis of the armature and having substantially nomoment of inertia with respect thereto andsaid damping means surroundingsaid axis and comprising a block of damping material having a majordimension extending at a right angle to the said axis of rotation.

12. In an electric signal translating device, the combination of anarmature member having a pivot shaft, a frame in which said pivot shaftis journalled, a cylindrical extension piece for the armature member` ablock of damping material mounted on and surrounding said extensionpiece, said block of damping material having dlmensions and shape suchthat its outer edges are relatively widely spaced from said cylindricalextension piece, an elongated torsional spring member connected with theshaft and extending axially therefrom into rigid connection with aportion of said frame. Y

- JAMES M. KENDALL.

